Magnetically controlled electron discharge amplifier circuits



March 20, 1951 c. R. KNIGHT 2,546,033

MAGNETICALLY CONTROLLED ELECTRON DISCHARGE AMPLIFIER cmcuns Filed June 7, 1946 2 Sheets-Sheet 2 Tigb.

lNFl/I' 2 a 4 4 25 2a 6; 1 E .23

v 23 26' 7 3/ "|fl.(f |\r -fi (ll/IP07 Inventor: Chester R- Knight,

b M 2% His Attorne g.

resistance.

Patented Mar. 20, 1951 UNITED; STATES PATENT OFFICE.

Chester R. Knight,.Scotia,.N. Y., assignor taGeneral Electric. Company, accorporationof New npmieenmnmne-v, 1946, Serial no; 674,945"

vide animproved amplifier-capable of' amplifying electric signals varying" at a very slow rate and which is inherently balanced so-thatzero output-voltage is-obtained with zero input voltage and. positive or- -negativeoutput voltage isobtainedin accordance withthe polarity of the input voltage.

A further :object of my invention is to-provide an improved amplifier ..capable. of. amplifying slowlyvarying. electric signals. but; which. is=;not sensitive. to "unpredictable...changes: in. contact .Another object of. this; invention is to.- provide an amplifier inwhich. the. input circuit is entirely isolated from the. output. circuit and which may .be arranged to provide any desired impedance transformation .by -means of simple changes in the circuit. components.

Still. another-object of. my inventionis to provide an amplifier capable of amplifying slowly varying voltages or currents and which maybe connected intandem with other amplifiers. of the same type withoutrequiringa separate source of operating voltage.

Yet another object of this invention is to provide animproved amplifier using electron discharge devices of the magnetically controlled type which'uti-l'izes simple" feedback means, which means may be arranged toprovide positiveor negative feedback.

Still another object" of myinvention is to provide an improved: electron discharge device wherein the space current may be magnetically controlled.

of the type to which this invention is applicable;

(c1. mar-1.71).

thedevi'ce of Fig. 1, Fig. 3 shows the-.circuitzdiagram of an amplifierwconstructed in: accordance .with this invention, Fig. .4 illustrates the-opera- .tion of the amplifier of Fig. 3, Fig. 5 shows a .two stage amplifier constructed in. accord with my invention, Fig. 6 showsza modified. electron discharge device of .the type shown. in :Fig. 1, Fig. 7 shows the circuit diagram of a. feedback amplifier constructed in accordance with invention, and Fig. 8' shows an oscillator constructed in accordance therewith.

Referring: now to. Fig. l', the electrondischarge device to which this invention isapplicablerconsists of a cylindrical anode 2 mountedsabout cathode I as an axis. Solenoid}. is-arrangedzto set up a. magnetic fieldwithin the. inter-electrode space of. the device, thisfieldi being: substantially alonggthe axis of cathode l. .Itis Well: known that in a device of thisntypeitheaincreased; magnetic field associated: with increasing: current-.fiow in coil 3 causes electrons:fromacathode :tatofollow a curved path. in. passingto. anode If the field strength sufiicient,:.theaelectronsenever reach the anode-andno space currentxfiow. takes place. Until this point isreached, however, the :spacecurrent flow is :nearlyequal :to ithe value determined byelectron emission and spacecharge and is independent; of magnetic flux.

A number of factors combine to cause. the.

. to be. cut-off along. somerportions' of. the; cathode while not limiting current flow. in' lother portions, thereby reducing the total: space path current flow but not preventing it entirely.

Fig. 3' shows an amplifier-constructed iniaccordance with the principles of this. invention. In the figure, 4 and 5 are electrondischargede- .vicesof the type-shown inEig. l,I-the:.ano.deiof each being connected to the-positive terminal. of unidirectional voltageesource. 6. The cathodes of devices 4 and .5 are connected toztermi-nals A and B respectively, these points being. connected to the :negative terminal of source lithrough potentiometer l. The magnetic field.Within-.-device 4 is determined by. the combined magneto.- motiveforce of; coils B and 91 and the magnetic field within device 5 determined by the. total magnetomotive forceof coils l3 and I4. Coil 8 is connected through variable resistance I10 to unidirectional voltage source 6 whereaszcoil 9 isconnected to input-terminals II and I2; 'iCOil 3 source 5 by means of variable resistance I5 while coil I3 is connected to terminals II and I2. Output voltage is taken across terminals I6 and I! which are connected to points A and B respectively.

Coils 8 and 9 and coils I3 and I4 are arranged in such fashion that current flow due to voltage applied across terminals II and I2 causes bucking magnetomotive force with respect to the fiux in one electron discharge device and a reinforcing magnetomotive force as to flux in the other device. If, for example, input terminal II is positive with respect to input terminal I2, coil 9 may have the same direction of magnetization as coil 8 whereas coil I3 magnetizes in a direction opposing coil I4. This condition is indicated by the arrows of Fig. 3. On the other hand, if terminal II is negative with respect to terminal I2, current flow in coils 9 and I3 is reversed and coil I3 aids coil I4 in establishing flux in device 5 and coil 9 opposes coil 8 in establishing fiux in device 4.

In operation, resistances I and I are adjusted until devices 4 and 5 are operating at substantially the mid-point of the region of space current cut-off. That is, the magnetomotive forces due to coils 8 and I4 are adjusted to produce flux densities within devices 4 and 5 of value corresponding to the dotted line 0-0, Fig. 2. With the devices operating in this region, any small change in total magnetomotive force, such as would be due to current flow in coils 9 and I3, has a maximum efiect on the space path current of each device. In addition, the position of the moving terminal of potentiometer 1 is adjusted until no voltage drop exists between points A and B, thereby compensating for any small differences in the space path current fiow of devices 4 and 5 producing zero output voltage for the condition of zero current fiow in coils 9 and I3.

Operation of the circuit of Fig. 3 when current is flowing in coils 9 and I3 is illustrated in Fig. 4. In this figure, curve I8 shows the space path current flow in device 4 as input terminal I I is made positive with respect to input terminal I2. This decreasing current flow results from the reinforcing action of magnetomotive force from .coil 9 which aids the flux produced by coil 8.

Curve I9 shows a similar curve for device 5, this a curve showing increasing current because the net flux in device 5 is reduced by the current flow in coil I3. In curves 29 and 2|, the corresponding voltage drops across the two portions of potentiometer I are shown, curve 20 being the voltage drop between point A and the moving terminal of potentiometer I and curve 2| being the voltage drop between point B and the moving terminal of potentiometer I. At zero input voltage, curves 20 and 2! cross by reason of the fact that potentiometer "I is adjusted to produce no output voltage for this condition. Curve 22, Fig. 4, shows the output voltage across terminals I5 and I1 corresponding to curves 20 and 2I. In this case the output voltage varies as the difierence between these two curves.

Inasmuch as the power for the voltage power change does not require that the input voltage be varying to produce an output voltage, the system acts as a direct current amplifier and reproduces all input voltage variations even though the rate of change is exceedingly small. Furthermore, the input circuits are completely separated from the output circuits, thereby providing a high degree of stability and ease of ad- J'ustment.

One of the advantages of the circuit of Fig. 3 resides in the good stability obtained. In conventional amplifiers designed to amplify slowly varying changes in a signal potential, contact potentials within the electron discharge devices cause output voltage changes having no relation to the applied signal voltage. This voltage varies as the electron discharge device ages and as the operating temperature varies. Avoiding the undesirable consequences of these changes has proven a difiicult problem, particularly when the signals of the order of a volt are to be am lified. In the circuit of Fig. 3, this difi'iculty is avoided because large cathode-anode space paths are provided in the electron discharge devices and contact potentials are not amplified.

In Fig. 5 a modified amplifier consisting of two stages is shown. The first stage, comprising devices 4 and 5 and having output terminals I6 and I7, is identical with that shown in Fig. 3, corresponding numerals indicating corresponding circuit components. The second stage, comprising devices 25 and 28 is of similar construction and has coils 26 and 21 connected to output terminals I6 and I! of the first stage. Resistance 24 is adjusted to cause current flow in coil 23 to be of value to cause device 25 to operate at line C-C, Fig. 2, and resistance 30 is adjusted to produce current flow in coil 29 to operate device 28 at the same condition. Coil 2'6 opposes coil 23 and coil 2! aids coil 29 for one direction of voltage across terminals I6 and I1 and the reverse action occurs for the other direction of voltage across terminals I6 and I1. Potentiometer 3I is adjusted to produce no output voltage across terminals 32 and 33 when no voltage exists at input terminals I I and I2.

One advantage of the tandem circuit shownin Fig. 5 is that only one source of unidirectional operating voltage is required for the complete system. This is due to isolationv of the input and output circuits of each stage and permits any number of amplifiers to be operated from the same voltage source. This is in marked contrast to direct current amplifiers using conventional electron discharge devices as such amplifiers ordinarily require a separate source of operating voltage for each stage.

As an indication of the circuit values that might be used in an amplifier such as that shown in Fig. 5, the following values have been successfully used:

Source 6, volts Coils 9 and I3, 28 layers #20 wire wound on a 3 inch form Coils 8, I4, 23 and 29, 30 layers #38 wire wound over coils '9, I3, 26, and 21 respectively Coils 26 and 21, 30 layers #40 wire Resistances I0, I5, 24, and 30, 6000 ohms (variable) Potentiometers 1 and 3|, 12,000 ohms Devices 4, 5, 24, and 28, General Electric type lurns Ourrent Resistance Amperes 1 '10 1 10 '1 10 100 0. l 100 1, 000 0.01 1,000 I 10', 000 '0. 001 10.000 100, 000 0. 0001 100, 000

Fr omthe standpoint of operation of the amplifier any of these coiloombinations may be used a'nd it is merely necessary-toselect=acoil having'resistance of optimumvalue -from the standpoint of-the source of-input voltage. If a highimpedance sourcers-uchas-=a-n electro cardiograph,- an encephalograph, or -aphotocell, is encountered, =-eoils having a-verygreat number of turns can be used. On-the-other hand,--if thesource has'low impedance and produces relatively large current at low voltage, a small number of turns can be used in the coils. In either event the remaining portions of the amplifier are identical and only the coils need be changed.

Fig. 6 shows the magnetically controlled electron discharge device of Fig. 1 modified to improve the amplification thereof when used in an amplifier such as shown in Fig. 3. An iron core comprising external member 34 and internal members 31 and 38 is provided to limit the length of the flux path in air to the active length of the space path of the electron discharge device, thereby reducing the magnetomotive force requirement to that actually required to produce fiux across space path of the electron discharge device. This produces a corresponding reduction in the magnetomotive force of the control coils and thus reduces the input power requirements. Electrical connections to cathode I can be made by a small hole 39 passing through members 37 and 34.

Another feature of the device shown in Fig. 6 is the screen grid electrode 36 placed between the cathode and anode. This electrode acts in a manner similar to the screen grid of a conventional electron discharge device, increasing the amplification factor and space path resistance and electrostatically isolating the anode from the cathode. Constant bias voltage is applied to this electrode from unidirectional voltage source 44. In addition a space charge or suppressor grid can be added to the device.

Fig. '7 shows a modification of the amplifier of Fig. 3 providing feedback effects. This is accomplished by coils 40 and 4! which are arranged to produce flux in the space path of electron discharge devices 4 and 5. If negative feedback is desired, these coils are wound to provide a magnetomotive force in opposition to the magnetomotive force of coils 9 and I3, thereby reducing the apparent input magnetomotive force and producing the greater fidelity, lower output impedance, improved stability, and other features of negative feedback amplifiers. Positive feed- 6 rbacka scanibea achieved by reversing themagnetomotive forces of ;these' coils; thus providingsgreateramplification andlincreasedi input-impedance at a sacrifice of stabilityand fidelity.

In Fig.1 8,:thei amplifier of Fig.2:3-is arranged as :a. feedback'oscillator. Control coi-lsai9 Land l3l are connected in series with napacitorfm'l and 'i-the a combined circuit ijconnected through 're'sistance '43 to :output terminals ifiwand :11. zFeedback 10 1 Voltage adjustment is .providedilby varying -gthe ":yalue vof resistance 43. l'With'ithis vicircuit,.ieoils .9 I and" l 3 are -wound: in directionss-suchi that-ithe .:?ma'gnetomotive' forces -:they :"produce: as -a result I of ia ipredetermined ip'olarity :"bet weenterminals a l6oand H is :of 'stheesame direction as lither magnetomotive fOI'CGS-"f required :to" produce 1 toutput '-voltage of that'tpolarityi if the systemiis' operated as-an amplifier. '=.With*resistance r4-3i.-adjusted.to provide :a :sufiicient amount--01? feedback, tos'cilla- 20615161182316 thus ;:produced :by the: circuit-.ofi-Fig. '8,

.zthe frequencyof the -;oscillations being-idetermined by the.naturalrresonantfrequencysofzcon- .denser 42: operating in connection-with coils 9 and-l3.

One of ithe features ofithe loscillator of =Fig.

Sis the low oscillating frequency'that-canb e-"robe ata'ined. '"Coils iii/and 13 may be chosenttoiihave :a very largeuvalue of inductance by usinga large .Jnumber 50f .turns of :small fsize- 'wire. This large inductance, 1 in :aco'njunetion with 1a reasonably large value of capacitor 42, provides a circuit having an exceedingly low resonant frequency (i. e. 10 cycles per second or less). The resultant oscillations may then be used to control devices such as electric signs where it is desired to provide a uniform periodic change at low frequency.

While I have shown and described my invention as applied to a particular system of connections and embodying various devices dia- 40 grammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing therefrom, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An amplifier comprising two electron discharge devices, each having an anode and a cathode, an output resistance directly connected between said cathodes, a source of unidirectional potential having a positive and a negative terminal, means for connecting the positive terminal of said source directly to both of said anodes, means for connecting the negative terminal of said source to a movable tap on said resistance, means for producing a magnetic flux in each of said devices of value corresponding to a region of relatively large space current change for small 0 change in magnetic flux comprising respective windings for each of said devices connected in parallel across said source, an input circuit including means to aid the flux produced in one device and means to oppose fiux produced in said 5 other device when applied voltage is of one polarity, and having the opposite effect when said applied voltage is of the opposite polarity, and an output circuit connected directly across the said resistance. 2. An amplifier comprising two electron discharge devices each having an anode, and a cathode, an output impedance coupled between each of said cathodes, means to supply space path voltage for said devices coupled between each of said anodes and an adjustable point on said output impedance, means to produce magnetic flux in each of said devices of magnitude corresponding to a region of relatively large space current change for small change in magnetic flux, an input circuit, said input circuit including means to aid flux from said second means in one device and to oppose flux from said second means in said other device when an applied voltage is of one polarity and having the opposite effect when said applied voltage is of opposite polarity, a utilization circuit connected directly across said output impedance and means to alter the flux in said devices in accordance with the voltage across said output impedance comprising respective windings for each of said devices connected in series across said output impedance.

3. A multistage amplifier having two pairs of electron discharge devices, each of said devices having an anode, and a cathode, a respective output resistance connected between the cathodes of each pair of said devices, a common source of unidirectional potential connected between each of said anodes, and respective taps on each of said resistances, respective windings for the devices in each pair to produce magnetic flux in each of said devices of magnitude corresponding to a region of relatively large change in space current for small change in magnetic flux, an input circuit, said input circuit including means to increase the flux of one of said devices of one pair and to decrease the flux in the other of said devices of said one pair in accord with the value of an applied electromotive force, means responsive to the difierence in space current flow between said last mentioned devices to increase the flux in one of the remaining of said devices and to decrease the flux in another of the remaining of said devices, a utilization device, and means to apply voltage to said utilization device in accordance with the difierencein space current flows of said last two mentioned devices.

CHESTER R. KNIGHT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,530,364 Hennelly Mar. 17, 1925 1,535,082 Alexanderson Apr. 21, 1925 1,552,219 Mercer Sept. 1, 1925 1,586,570 Nichols June 1, 1926 1,593,373 Vander Bijl July 20, 1926 1,816,682 Langmuir July 28, 1931 2,013,093 Frantz Sept. 3, 1935 2,071,923 Frantz Feb. 23, 1937 2,296,764 Braden Sept. 22, 1942 2,432,748 Glass Dec. 16, 1947 

